A MISFET (Metal Insulator Semiconductor Field Effect Transistor) can be formed by forming a gate insulating film on a semiconductor substrate, forming a gate electrode on the gate insulating film and forming source/drain regions by ion-implantation or the like. As the gate electrode, a polysilicon film is typically used.
In recent years, however, the gate insulating film has been thinned with the miniaturization of a MISFET element, and the influence of depletion of the gate electrode when a polysilicon film is used for the gate electrode has become unignorable. For its solution, there is a technique of suppressing a depletion phenomenon of the gate electrode by using a metal gate electrode as the gate electrode.
Further, when the gate insulating film has been thinned with the miniaturization of the MISFET element and a thin silicon oxide film is used as the gate insulating film, electrons flowing through a channel of the MISFET tunnel through a barrier made of the silicon oxide film to flow in the gate electrode, and a so-called tunnel current is generated. For its solution, there is a technique of reducing a leakage current by using a material having a dielectric constant higher than that of the silicon oxide film (high-dielectric material) as the gate insulating film to increase a physical film thickness without changing a capacitance.
V. Narayanan et al., VLSI 2006, p. 224, P. Sivasubramani et al., VLSI 2007, pp. 68-69, H-S. Jung et al., VLSI 2005, pp. 232-233 and M. Inoue et al., VLSI 2009, pp. 40-41 (Non-Patent Documents 1 to 4) describe techniques for a MISFET having a metal gate electrode and a high-dielectric gate insulating film.